Interpretive Summary: Downy brome is a difficult-to-control weed in winter wheat in the western United States. Presently, it can not be controlled consistently with commercial herbicides. In recent years, natural products from naturally-occurring soil bacteria have been shown to be potentially useful as bioherbicides. Toxins produced by one such bacterium, Pseudomonas fluorescens strain D7 (strain D7), severely inhibited root growth of downy brome in a seed germination test at very low concentrations (80% inhibition at about 2 ng total solids/ml). Shoots were 200 times less sensitive than roots. In tests conducted with developing seedlings in liquid nutrient solutions, downy brome also was inhibited, but at concentrations much higher than those required in germination tests (dry weight reduced 50% by toxin concentrations of 150,000 ng total solids/ml), and downy brome was almost twice as sensitive as winter wheat to the toxin. Toxins also inhibited plant water uptake, disrupted membranes of isolated downy brome cells, and in a purified form produced from two different food sources, stopped growth of downy brome seedlings immediately, and killed the plants. These results show that the Strain D7 toxins may be able to selectively injure downy brome but not wheat, that highly active toxins are produced when Strain D7 bacteria are grown on several different food sources, and that they are especially harmful to roots of germinating downy brome seeds, as well as to whole plants and isolated cells.

Technical Abstract:
Toxins produced in shake cultures by Pseudomonas fluorescens strain D7 (strain D7), a naturally occurring rhizobacteria, inhibited root and shoot growth of downy brome by 80% in a seed germination agar assay at concentrations of 0.0002 and 0.04% of the initial concentration in cell-free broth, respectively. These concentrations were estimated to be 20 and 400 ng total solids/ml, respectively. In a hydroponics assay, downy brome and winter wheat dry weights were reduced 50% by cell-free broth concentrations of 15 and 26% of the initial broth concentration, respectively, indicating an almost 2X selectivity between winter wheat and downy6 brome. Plant water use was reduced similarly Plasma membranes of isolated protoplasts of downy brome were damaged within 2 h by diluted cell-free broth. Cold-precipitated toxins of strain D7 produced in semi-synthetic and synthetic media reduced dry weight of downy brome seedlings by 100% and 89% respectively, and killed plants rapidly. Strain D7 toxins derived from several media sources adversely affected downy brome at several growth stages and affect seeds, whole plants, and isolated cells.